Pitch plays a fundamental role in auditory and speech perception. It defines melody and harmony in music; it contributes both prosodic and (in tone languages) lexical information to speech; and it plays a crucial role in our ability to segregate and follow competing sound sources. Despite its importance, many basic aspects of pitch processing and perception remain unknown. For instance, although hearing loss often degrades pitch perception, there is little agreement as to the underlying mechanisms. In addition, despite cochlear-implants? many successes involving speech perception, pitch remains very poorly represented. These deficits may help explain the difficulties people with hearing loss and cochlear implants experience when listening to speech in complex acoustic environments. The long-term goal of this project is to improve our understanding of the mechanisms underlying the perception of pitch of single and concurrent sounds in the normal and impaired auditory systems. The proposal is divided into three specific aims. The first aim uses perceptual measures to test important hypotheses regarding the coding of pitch of pure and complex tones in isolation and in mixtures. In the first experiment, novel applications of studies of individual differences are used to test the hypothesis that ageing and cochlear hearing loss contribute independently and via different mechanisms to the degradation of pitch perception. The second experiment addresses questions of how multiple pitches are coded under normal-hearing conditions. The third and fourth experiments test simulated and real cochlear- implant users, respectively, to probe the limits of spectral resolution necessary to extract complex pitch in future generations of implants, and to test the extent to which current implants can transmit multiple pitches simultaneously. The second aim involves three high-field (7 Tesla) functional magnetic imaging (fMRI) studies. One study provides the first test of tonotopy up to very high frequencies, to test the hypothesis that poorer pitch perception at high frequencies reflects cortical rather than peripheral coding limitations. Another study attempts to separate representations of pitch from timbre by dissociating fundamental frequency and spectral centroid. The third study addresses a potential stimulus confound involving spectral sparsity that may have influenced previous attempts to locate a ?pitch center? within the auditory cortex. The third aim proposes a large-scale study of the effects of musical and pitch training on skills that are either directly related to music, such as pitch discrimination, or are more general, such as the ability to understand speech in noise. Earlier studies have reached mixed conclusions on this important question, in part because of the relatively small sample sizes tested. One large-scale correlational study involving a comparison of perceptual and physiological measures in people with or without long-term musical training, and one large-scale randomized controlled study of the effects of pitch-discrimination training in adults will provide more definitive answers to these important questions.